Acute infective endocarditis is a life threatening disease characterized by the involvement of normal valvular endothelium. The predominant pathogen in this disease is Staphylococcus aureus. It has been theorized that the unique ability of staphylococc to colonize and invade normal valvular endothelium is responsible for this association. In the present proposal S. aureus attachment to endothelial cells will be studied using an in vitro system which simulates the valvular surface colonized by bacteria in acute infective endocarditis. These cells, harvested from human valvular tissue, will be used to explore the surface structures which mediate staphylococcal attachment. The ability of different blood culture isolates to adhere to endothelial cells harvested from different vascular sites will be compared. The number of potential binding sites and the rate of attachment and endocytosis will be investigated. Transmission electron microscopy will be used in morphologic studies which will determine the specificity of endocytosis and the fate of intracellular bacteria. The ability of specific endothelial cell membrane components to bind S. aureus and potentially act as mediators of adherence will be studied. Binding of radiolabelled, solubilized membrane proteins and glycoproteins to S. aureus will be assessed. Solubilized surface membrane proteins and glycoproteins will be further purified and characterized by affinity chromatography. The potential role of these membrane components in staphylococcal adherence will be assessed in competitive adherence assays. In this proposal we will investigate whether selective staphylococcal adherence to normal valvular endothelium contributes to the predominance of this organism as a pathogen in acute endocarditis. These studies will increase our understanding of what remains a life-threatening disease.